The largest difference between the two diagrams is the
simplification of the intake and exhaust strokes in the ideal cycle
. In the exhaust stroke, heat Q
out
is ejected into the environment. In a real engine, the gas leaves the engine and is replaced by a new mixture of air and fuel.
What is the difference between actual cycle and air standard cycle?
| Air Standard Cycle Actual Cycle | Pure air is used. Working fluid is a combination of air fuel and residual gas in the cylinder. | Composition of working fluid remains constant. Composition of working fluid changes at every instant of time. |
|---|
What is an ideal engine cycle?
Ideal cycles are
simplified thermodynamic closed cycles to analyze the compression, combustion, and expansion process in an engine with a focus on extraction of work from combustion of the fuel–air mixture
.
What are actual cycles?
What is the Actual cycle? The Actual cycle in the Thermodynamics is a cycle of operations experienced by the actual Internal combustion engine where the efficiency of this actual Internal combustion engine is much lower than the Ideal cycle or Air-Standard cycle due to various losses in the processes.
Why the actual cycle efficiency is much lower than the air standard cycle?
The actual cycle (see Fig. (3) experienced by internal combustion engines is an open cycle with changing composition, actual cycle efficiency is much lower than the air standard efficiency
due to various losses occurring in the actual engine
. Air cycle with variable specific heat and dissociation.
What losses are considered in the actual cycle and describe those losses in detail?
Time loss
occurs due to time required for mixing of air and fuel and time required for complete combustion of the mixture. Time loss depends upon type of fuel used, air-fuel ratio and flame velocity in the combustion chamber. Heat loss occurs due to transfer of heat through water jackets and cooling fins.
Which engine produces more power for same compression ratio?
2 stroke diesel engine
is the most efficient engine.
What is fuel Aircycle analysis?
Fuel-air analysis is
more realistic analysis when compared to Air-standard cycle analysis
. An accurate representation of constituents of working fluid is considered. More accurate models are used for properties of each constituents.
What is ideal engine?
Definition of ideal engine
:
a heat engine operating on a reversible cycle
(as a Carnot cycle)
Which of the following power cycle is ideal cycle?
The
Rankine cycle
is the ideal cycle for vapor power plants; it includes the following four reversible processes: 1-2: Isentropic compression Water enters the pump as state 1 as saturated liquid and is compressed isentropically to the operating pressure of the boiler.
What is ideal cycle efficiency?
Modern engines have compression ratios in the range 8 to 11, resulting in ideal cycle efficiencies of
56% to 61%
.
Which cycle is used in diesel engine?
Diesel combustion. The diesel engine is an intermittent-combustion piston-cylinder device. It operates on either a
two-stroke or four-stroke cycle
(see figure); however, unlike the spark-ignition gasoline engine, the diesel engine induces only air into the combustion chamber on its intake stroke.
How are SI and CI engine fuels rated?
Octane number is considered for SI engine while Cetane number is considered for CI engine
. They are calculated through comparison by perfect fuel characteristics. Octane number and Cetane numbers are used to rate fuels in IC engines.
What are the major losses in actual cycle?
These losses are as follows: 1-
Losses due to variation of specific heats with temperature
: already discussed. 2- Losses due to dissociation: already discussed. 3- time losses: In theoretical cycles the burning is assumed to be instantaneous.
What is the difference between air standard assumptions and the cold air standard assumptions?
♨Along with all the above assumptions, taking constant specific heats at room temperature, at 25 C for air, is called cold air standard assumptions. ✔
Assuming constant specific heats at room temperature 25 C for air
is only the difference between air standard assumptions and cold-air standard assumptions.
Why is Otto cycle more efficient?
Although for a given compression ratio the Otto cycle has higher efficiency,
because the Diesel engine can be operated to higher compression ratio
, the engine can actually have higher efficiency than an Otto cycle when both are operated at compression ratios that might be achieved in practice.
Which one of the following processes is the major difference between the air standard Otto cycle and the air standard Diesel cycle?
All processes are ideal, combustion is replaced by heat addition to the air, and exhaust is replaced by a heat rejection process which restores the air to the initial state. The most significant difference between the ideal Otto cycle and the ideal Diesel cycle is
the method of igniting the fuel-air mixture
.
When the engines are built to withstand same thermal and mechanical stresses which of the cycle has highest air standard efficiency?
The cycle which has the least heat rejected will have the highest efficiency. Thus, the
Otto cycle
is the most efficient and the Diesel cycle is the least efficient of the three cycles.
During which part of cycle heat loss are significant explain?
among those, the Heat loss will occur
during the Combustion and the expansion strokes
. Heat loss during the combustion process definitely has a high influence on cycle efficiency. Whereas the heat loss during the end of expansion stroke will have effectively less influence on the cycle efficiency.
What are the limitations of air standard cycle?
All processes in cycle as internally reversible.
No internal heat generation. heat is added & rejected externally. The cycle is closed & processes are non-flow processes
.
Which cycle is more efficient for the same compression ratio?
Detailed Solution. For same compression ratio, the efficiency of an
Otto cycle
is higher than that of the diesel cycle.
Why diesel engine is usually more efficient than a spark ignition engine?
As the compression ratio increases, we got more compressed air in the cylinder which results in an increase in the thermal efficiency of the diesel engine
even though the thermodynamic cycle of the diesel engine is less efficient than the thermodynamic cycle of SI engine.
Which cycle is more efficient for the same compression ratio and heat input?
Explanation:
Otto cycle
is more efficient than Diesel cycle for a given compression ratio. 6. For constant maximum pressure and heat input, the air standard efficiency of the gas power cycle is in the order.
What is dissociation in IC engine?
Dissociation is
a process of disintegration of combustion products at high temperature
. This can also be considered as reverse process to combustion. During dissociation , heat is absorbed where as during combustion heat is released.
What are engine performance parameters?
Engine performance parameters are
power, torque and specific fuel consumption
. Brake torque is normally measured with a dynamometer – engine is mounted on a test bed and the shaft is connected to the dynamometer rotor.
What are the assumptions in fuel-air cycles?
1-
The working fluid is air, which continuously circulates in a closed loop (cycle)
. Air is considered as ideal gas. 2- All the processes in (ideal) power cycles are internally reversible. 3- Combustion process is modeled by a heat-addition process from an external source.
